U.S. patent number 9,114,357 [Application Number 14/486,395] was granted by the patent office on 2015-08-25 for treatment of nitrogen oxides in flue gas streams.
This patent grant is currently assigned to PeroxyChem LLC. The grantee listed for this patent is PeroxyChem LLC. Invention is credited to Philip A. Block, Timothy Holtz, Elena Pisanova, John M. Rovison, Jr..
United States Patent |
9,114,357 |
Block , et al. |
August 25, 2015 |
Treatment of nitrogen oxides in flue gas streams
Abstract
A method for removing nitrogen oxides in stationary source
combustion flue gas streams includes introducing a reagent or a
mixture comprising carbamide peroxide and water as a reducing
agent.
Inventors: |
Block; Philip A. (Garnet
Valley, PA), Holtz; Timothy (N. Tonawanda, NY), Pisanova;
Elena (Amherst, NY), Rovison, Jr.; John M. (Sanborn,
NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
PeroxyChem LLC |
Philadelphia |
PA |
US |
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Assignee: |
PeroxyChem LLC (Philadelphia,
PA)
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Family
ID: |
52666371 |
Appl.
No.: |
14/486,395 |
Filed: |
September 15, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150078978 A1 |
Mar 19, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14026338 |
Sep 13, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D
53/81 (20130101); B01D 53/79 (20130101); B01D
53/565 (20130101); B01D 53/56 (20130101); B01D
2251/206 (20130101); B01D 2251/106 (20130101); B01D
2257/404 (20130101) |
Current International
Class: |
B01D
53/56 (20060101) |
Field of
Search: |
;423/235 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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57-87816 |
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Jun 1982 |
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JP |
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08-332341 |
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Dec 1996 |
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JP |
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2006-110526 |
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Apr 2006 |
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JP |
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10-0197287 |
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Jun 1999 |
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KR |
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10-2002-0031426 |
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May 2002 |
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KR |
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Other References
International Search Report and Written Opinion issued on Mar. 19,
2015 for corresponding PCT Application No. PCT/U52014/055630. cited
by applicant .
Zamansky et al., "Removal of Multiple Air Pollutants by Gas-Phase
Reactions of Hydrogen Peroxide," in Preprints of Papers, American
Chemical Society (ACS), Div. of Fuel Chemistry; Journal vol. 40;
Issue No. 4; Conference 210, Natl. Meeting of the ACS, Chicago, IL,
Aug. 20-25, 1995, pp. 1039-1044. cited by applicant .
Cooper, D.A., "The influence of ammonia and hydrogen peroxide
addition on NO (Nitrogen Oxide) emissions in the flue gas channel
of a 16 MW coal-fired fluidized bed combustor," Journal of the
Institute of Energy, Jun. 1988, p. 78-84. cited by applicant .
Javed, Tayyeb, M., "Control of combustion-generated nitrogen oxides
by selective non-catalytic reduction," Journal of Environmental
Management, 83 (2007) p. 251-289. cited by applicant.
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Primary Examiner: Vanoy; Timothy
Attorney, Agent or Firm: Duane Morris LLP
Parent Case Text
PRIORITY CLAIM AND CROSS-REFERENCE
This application claims the benefit of U.S. application Ser. No.
14/026,338, filed Sep. 13, 2013, which application is expressly
incorporated by reference herein in its entirety.
Claims
What is claimed is:
1. A method of removing nitrogen oxides from a flue gas stream
comprising introducing a reagent comprising carbamide peroxide and
water into a stationary-source NO.sub.X-containing combustion flue
gas stream at a temperature between about 300.degree. C. to about
800.degree. C. in an amount sufficient to react with NO in the flue
gas stream, to yield a flue gas stream having a reduced
concentration of NO.
2. The method of claim 1, wherein the water is an active component
of the reduction process and the molar ratio of water to carbamide
peroxide is in the range of from about 5:1 to about 500:1.
3. The method of claim 2, wherein the optimal molar ratio of water
to carbamide peroxide is in the range of from about 50:1 to about
200:1.
4. The method of claim 1, wherein the carbamide peroxide is
introduced into the flue gas stream in an amount that provides a
molar ratio of the carbamide peroxide to the NO in the flue gas
stream being in the range of from about 0.2:1 to from about
5:1.
5. The method of claim 1, wherein the carbamide peroxide and the
water are introduced into the flue gas stream as a combined aqueous
solution via one or more spray nozzles.
6. The method of claim 5, wherein the carbamide peroxide and the
water are introduced separately and concurrently into the flue gas
stream, as a first stream containing solid carbamide peroxide and a
second stream containing water.
7. The method of claim 1, wherein the reagent is introduced into
the flue gas stream in an amount that provides at least 100 ppm of
urea for the reaction with the NO in the flue gas stream being
treated.
8. The method of claim 1, wherein the reagent is introduced into
the flue gas stream in an amount that provides at least 100 ppm of
H.sub.2O.sub.2 for the reaction with the NO and urea in the flue
gas stream being treated.
9. The method of claim 1, wherein the reagent is introduced into
the flue gas stream in an amount that provides at least 1000 ppm of
H.sub.2O in the flue gas stream being treated.
10. The method of claim 1, wherein flue gas stream temperature
during the carbamide peroxide injection is in the range of from
about 350.degree. C. to about 750.degree. C.
11. The method of claim 1, wherein flue gas stream temperature
during the carbamide peroxide reagent injection is in the range of
from about 400.degree. C. to about 550.degree. C.
12. The method of claim 1, wherein the reagent comprises a mixture
of carbamide peroxide, water, and urea.
13. The method of claim 12, wherein the molar ratio of the
carbamide peroxide to the urea to the water is in the range of from
about 0.25:1:50 to about 20:1:1000.
14. The method of claim 1, wherein the reagent comprises a mixture
of carbamide peroxide, water, and H.sub.2O.sub.2.
15. The method of claim 14, wherein the molar ratio of the
carbamide peroxide to the H.sub.2O.sub.2 to the water is in the
range of from about 0.5:1:50 to about 10:1:500.
16. The method of claim 1, wherein the reagent is employed as the
reducing agent in the absence of a catalyst used in Selective
Catalytic Reduction operations carried out on NO.sub.X-containing
flue gas streams.
Description
FIELD OF THE INVENTION
The present invention relates to the treatment of a combustion flue
gas stream from a stationary source to remove nitrogen oxide (NO)
before the gas stream is released into the atmosphere.
BACKGROUND OF THE INVENTION
Combustion of fuels such as coal, coke, natural gas or oil
typically results in the presence of pollutants in the combustion
flue gas stream resulting from the combustion process or derived
from impurities present in the fuel source. Electric utility power
plants that burn coal are a significant source of such combustion
process air pollutants, but other stationary fuel-burning
facilities such as industrial boilers, waste incinerators, and
manufacturing plants are also pollution sources.
The primary air pollutants formed by these stationary high
temperature combustion sources are sulfur oxides (e.g. SO.sub.2 and
SO.sub.3), also called SO.sub.X gases, and nitrogen oxides, also
called NO.sub.X gases, both of which are acid gases. Other
combustion pollutants of concern in these combustion flue gases
include other acid gases such as HCl and HF, Hg (mercury), CO.sub.2
and particulates. These individual pollutant components from
stationary combustion sources have been subject to increasingly
more stringent regulatory requirements over the past three decades,
and emission standards are likely to be tightened in the
future.
The removal or significant reduction of SO.sub.X and NO.sub.X
contaminants, as well as other acid gases and elemental mercury,
requires an integrated air pollution control system. Such
integrated air pollution control systems represent a particular
challenge in situations requiring retrofitting of first-time or
additional or enhanced pollution control measures, e.g., older
coal-fired electric power plants without any desulfurization
measures or power plants with SO.sub.X controls requiring
modifications for control of NO.sub.X gas emissions.
Nitrogen oxide or nitric oxide (NO) and smaller amounts of nitrogen
dioxide (NO.sub.2) are the normal constituents of NO.sub.X
contaminants formed in the combustion of fossil fuels like coal,
coke and oil. The presence of NO.sub.X in a flue gas stream
discharged to the atmosphere can result in a "brown plume" and is a
contributor to ground-level ozone pollution ("smog") and to
acidifying nitrate deposition.
The wet scrubbing desulfurization techniques utilized for SO.sub.X
removal from combustion flue gas are largely unsuccessful for
removal of NO that is also present since the latter has low water
solubility and is not amenable to aqueous alkali desulfurization
scrubbing techniques. Although NO.sub.X formation can be controlled
to some extent by modifying combustion conditions, current
techniques for NO.sub.X removal from combustion flue gas normally
utilize post-combustion treatment of the hot flue gas by Selective
Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction
(SNCR)
The Selective Catalytic Reduction procedure utilizes a catalytic
bed or system to treat a flue gas stream for the selective
conversion (reduction) of NO.sub.X to N.sub.2. The SCR procedure
normally utilizes ammonia or urea as a reactant that is injected
into the flue gas stream upstream, prior to their being contacted
with the catalyst. SCR systems in commercial use typically achieve
NO.sub.X removal rates of 80-90%, but improved catalyst systems
reportedly provide over 90% removal rates.
The Selective Non-catalytic Reduction procedure is analogous to SCR
except that no catalyst is employed in the treatment of a flue gas
stream with ammonia or urea for the selective reduction of NO.sub.X
to N.sub.2. High treatment temperatures are required for the
reduction reaction in SNCR, in the absence of the catalysts used in
SCR systems. SNCR systems are favored for retrofit of smaller
electric power utility plants because of their simplified
installation and modest equipment requirements. Drawbacks to
commercial SNCR systems are their requirement of very high and
narrow temperature ranges to effect conversion of NO.sub.X to N,
which often results in undesirable reaction byproducts, and their
NO.sub.X removal rates of only 30-70%.
Many individual approaches are described in the prior art for the
removal of specific SO.sub.X and NO.sub.X components. In actual
commercial practice, the engineering challenge is the design of an
integrated air pollution control system that can be retrofitted to
existing fossil-fuel fired electric utility plants that are in need
of updated or upgraded pollution controls for one or more of
SO.sub.2, SO.sub.4, NO, NO.sub.2, Hg, HCl, HF, CO.sub.2 and
particulates. Since individual electric utility plants are rarely
alike, retrofit systems need to be adaptable to the specific
requirements and needs of the electric utility plant being
modified.
The present invention provides an air pollution retrofit system
that is particularly well suited to (but not limited to) improving
the performance of removing NO.sub.X in SNCR-treated combustion
flue gas streams, utilizing a NO.sub.X reduction reagent. The novel
NO.sub.X treatment system of this invention is not taught in prior
art treatments for abating SO.sub.X and NO.sub.X contaminants in
combustion flue gas streams.
U.S. Pat. No. 4,213,944 of Azuhata et al. (Hitachi) discloses a
process for removing nitrogen oxides from a hot gas stream
containing the same by adding a reducing agent, preferably ammonia,
and hydrogen peroxide into hot gas stream at a temperature of
400-1200.degree. C. to decompose the nitrogen oxides to nitrogen
gas and water. The hydrogen peroxide is added concurrently with the
ammonia and is said to increase the activity of the ammonia,
particularly at gas temperatures of 400-800.degree. C., by
decomposing the ammonia to make it reactive with the NO.sub.X.
Sufficient hydrogen peroxide is added with the ammonia so that
excess unreacted ammonia is also decomposed.
A NO.sub.X treatment technique analogous to that described in the
Azuhata patent involves treatment of a combustion flue gas
containing NO.sub.X with ammonia and hydrogen peroxide and is
described by D. A. Cooper in "The Influence of Ammonia and Hydrogen
Peroxide Addition on NOx Emissions in the Flue Gas Channel of a 16
MW Coal-fired Fluidised Bed Combustor," Journal of the Institute of
Energy, vol. 61, no. 447 (1988), pp. 78-84.
U.S. Pat. Nos. 5,120,508, and 4,783.325 of Jones (Noell) disclose
methods of converting NO to NO.sub.2 in a flue gas stream by
injecting a gas containing a peroxyl initiator and oxygen into the
NO-containing gas stream. The peroxyl initiator is preferably
propane but may also be other hydrocarbons or hydrogen peroxide or
hydrogen. The resultant NO.sub.2-containing gas stream is then
treated in an absorption section to remove NO.sub.X and SO.sub.X
with a dry sorbent such as nahcolite or trona, the dry sorbent
being captured in a baghouse before the treated gas stream is
discharged to the atmosphere.
U.S. Pat. No. 5,670,122 of Zamansky et al. (Energy &
Environmental Research) discloses a method for removing NO,
SO.sub.3, CO, light hydrocarbons and mercury vapor (Hg) from
combustion flue gas by injecting into the gas stream atomized
droplets of either hydrogen peroxide or a mixture of hydrogen
peroxide and methanol, to convert the respective gas contaminants
to NO.sub.2, SO.sub.2, CO.sub.2 (for the CO and light hydrocarbons)
and HgO. The treatment is carried out at a gas temperature of about
377.degree. C. to about 827.degree. C., and the reaction products
are subsequently removed in a downstream scrubbing operation. The
treatment also may be carried out in combination with SNCR NO.sub.X
reduction technology, with the SNCR-treated combustion gas stream
being treated downstream with the H.sub.2O.sub.2 or
H.sub.2O.sub.2/CH.sub.3OH injection treatment. The method is also
described by Zamansky et al. in Preprints of Papers, American
Chemical Society (ACS), Div. of Fuel Chemistry; Journal Vol. 40;
Issue No. 4; Conference 210, Natl. Meeting of the ACS, Chicago,
Ill., 20-25 Aug. 1995, pp. 1039-1044.
U.S. Pat. No. 6,676,912 of Cooper et al. (NASA) discloses a method
of removing NO from stationary combustion gas streams by injection
of H.sub.2O.sub.2 into the gas stream to oxidize NO to NO.sub.2 and
HNO.sub.3 and HNO.sub.2, which species are more readily recovered
via aqueous wet scrubbing. The nitrogen acids and residual NO.sub.2
are then removed via wet scrubbing with water or an aqueous
alkaline medium or via passage of the flue gas stream through a
particulate alkaline sorbent in a baghouse. The method may
optionally include a preliminary flue gas desulfurization scrubbing
step to remove SO.sub.2, prior to the H.sub.2O.sub.2 injection.
U.S. Pat. No. 6,676,912 of Cooper et al. is hereby incorporated by
reference for its disclosures about the reaction of H.sub.2O.sub.2
and NO.sub.X and related reactions.
U.S. Pat. No. 8,147,785 of Pfeffer et al. (FMC) describes a method
for removing residual unreacted ammonia (ammonia slip) that is
present in a combustion flue gas stream that has been treated via
SCR or SNCR NO.sub.X treatment systems using ammonia or urea. The
flue gas stream is treated downstream of the SCR or SNCR operation
with aqueous hydrogen peroxide to remove residual unreacted
ammonia.
The present invention provides a method for the removal of NO.sub.X
in a SNCR system that enables the removal of NO.sub.X from the gas
stream at reduced operating temperatures and that does not require
use of ammonia as reducing agent.
SUMMARY OF THE INVENTION
In accordance with the present invention, nitrogen oxides are
removed from a flue gas stream in a method comprising introducing a
reagent or a mixture comprising carbamide peroxide and water into a
stationary-source NO.sub.X-containing combustion flue gas stream at
a temperature between about 300.degree. C. to about 800.degree. C.
The reagent or the mixture is in an amount sufficient to react with
nitrogen oxides (NO.sub.X) such as NO in the flue gas stream, to
yield a flue gas stream having a reduced concentration of nitrogen
oxides (NO.sub.X) such as NO.
BRIEF SUMMARY OF THE DRAWINGS
FIG. 1 is a graph that shows the results of the study described in
Comparative Example 4, in which hydrogen peroxide alone was used to
treat a gas stream containing NO. FIG. 1 shows concentrations of
NO, NO.sub.2 and NO+NO.sub.2 (NO.sub.X) in the
H.sub.2O.sub.2-treated gas stream as a function of time during the
study.
FIG. 2 is a graph that shows the results of the study described in
Example 5, in which carbamide peroxide aqueous solution was used to
treat a gas stream containing NO. FIG. 2 shows concentrations of
NO, NO.sub.2 and NO+NO.sub.2 (NO.sub.X) in the carbamide peroxide
treated gas stream as a function of time during the study.
FIGS. 3-7 are graphs that shows the results of the study described
in Examples 6 and 7, in which carbamide peroxide (FIGS. 5 and 7)
and urea alone (FIGS. 3, 4, and 6), were used to treat gas streams
containing NO. The Figures show concentrations of NO, NO.sub.2 and
NO+NO.sub.2 (NO.sub.X) in the reagent-treated gas streams as a
function of time during the studies.
FIG. 8 is a schematic flow diagram illustrating a preferred
embodiment of the combustion flue gas NO.sub.X treatment process of
this invention that is described in Example 10.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a treatment method for removing nitrogen
oxides (NO.sub.X) from a combustion flue gas stream from a
stationary combustion source, e.g., an electric power plant
generating facility, by conversion of the nitrogen oxides,
particularly NO, to water and nitrogen gas using a two component
reagent that serves as the reducing agent.
The invention is particularly useful for effecting the conversion
(reduction) of nitrogen oxides at relatively low temperatures,
without the need for a catalyst such as is typically used in SCR
operations to effect reduction of NO.sub.X. The NO.sub.X treatment
method of this invention accomplishes this objective using a
reagent that is urea in combination with hydrogen peroxide such as
carbamide peroxide, in the absence of a catalyst, such as the
catalysts typically utilized in SCR operations. Carbamide peroxide
[(NH.sub.2).sub.2CO.H.sub.2O.sub.2] is a solid adduct of 1 mole of
Hydrogen peroxide with 1 mole of urea. The reagent comprising
carbamide peroxide and water is injected or otherwise introduced
into a flue gas containing NO.sub.X such as NO to effect removal
(conversion) of a substantial portion of the NO.sub.X in the
treated gas stream, as is described in more detail below. The
reagent can comprises a mixture (such as a solution or a slurry) of
carbamide peroxide and water prepared before introduced into a flue
gas containing NO.sub.X. The mixture can further comprise
additional urea or hydrogen peroxide. The carbamide peroxide and
the water can be also introduced separately and concurrently into
the flue gas stream, as a first stream containing solid carbamide
peroxide and a second stream containing water.
The invention is particularly useful for modifying the operation of
conventional Selective Non-Catalytic Reduction reactor systems to
be adapted to operation at reduced temperature ranges, e.g., below
800.degree. C., using the urea-based reagent of this invention
instead of ammonia as the reducing agent.
Combustion Flue Gas Stream
The combustion flue gas stream exiting the combustion zone of a
stationary source contains a variety of components that are
desirably reduced or removed from the flue gas prior to its being
discharged to the atmosphere, among which are the NO.sub.X
components treated according to the present invention. The precise
composition of the combustion flue gas depends primarily on the
nature of the fuel and on the furnace design and operating
parameters. For example, the fuel may be, e.g., coal, oil, coke or
natural gas, etc., and in the case of coal, coal may be high sulfur
or low sulfur, bituminous or anthracite, etc.
A representative flue gas stream obtained from combustion of high
sulfur coal containing 2.5 wt. % sulfur, burned using 10% excess
air, has the composition shown in Table 1.
TABLE-US-00001 TABLE 1 Flue Gas Composition Component
Concentration: volume basis NO 350-400 parts per million (ppm)
NO.sub.2 10-20 ppm SO.sub.2 0.22%.sup. SO.sub.3 20 ppm H.sub.2O 9%
CO.sub.2 15% Hg 1 part per billion (ppb) Other Gases 76%
The NO concentration in the flue gas stream is typical of that
expected from the burning of high sulfur coal in a furnace that is
not equipped with low NO.sub.X burners. The NO.sub.2 concentration
typically represents about 5% of the total NO.sub.X. The SO.sub.2
concentration in the flue gas stream is relatively high, as would
be expected from the burning of high sulfur coal. The SO.sub.3
concentration is typically only about 1% of the SO.sub.2
concentration.
The foregoing flue gas composition is simply meant to be
illustrative of a typical combustion flue gas stream. The present
invention is adapted to be used with a wide range of different flue
gas compositions from a variety of stationary sources and air
pollution control systems, within the parameters described in more
detail below for the method of this invention.
Carbamide Peroxide Reagent
The NO.sub.X treatment method of the present invention involves use
of carbamide peroxide, in combination with water, as the reagent to
effect reduction of NO.sub.X compounds present in the flue gas
stream being treated.
The reagents, carbamide peroxide and water, are injected or
otherwise introduced into the flue gas stream, preferably in
aqueous form, e.g., as an aqueous solution or an aqueous
slurry.
The reagent components are preferably combined prior to their
injection or introduction into the flue gas stream. Alternatively,
the carbamide peroxide and water components may be introduced
separately but concurrently into the flue gas stream being treated
provided that good mixing of the two reagent components is ensured.
In some embodiments, the carbamide peroxide component can be used
in dry powder form when introduced into the flue gas stream.
For purposes of the present specification, the term aqueous reagent
solution should be understood as meaning that the reagent
components that are normally solids (urea or carbamide peroxide)
are totally solubilized in the aqueous solution. The term aqueous
reagent slurry, on the other hand, should be understood as meaning
that the reagent components that are normally solids (urea or
carbamide peroxide) may only be partially solubilized and that a
portion of such solids are suspended as solids (preferably as
finely-divided particulates) in the aqueous slurry. The terms
aqueous reagent medium and aqueous medium should be understood to
include or encompass both solutions and slurries.
Carbamide peroxide is a solid adduct, which contains both urea and
hydrogen peroxide, present in a 1:1 molar ratio.
Carbamide peroxide is preferably injected or introduced into the
flue gas stream in aqueous form, e.g., as an aqueous solution. The
solubility of carbamide peroxide in water is relatively high, e.g.,
about 50 g. carbamide peroxide/100 ml, water at 20.degree. C.
Alternatively, carbamide peroxide may be introduced as a solid into
the flue gas stream, e.g., as a finely-divided particulate solid or
powder, or as a concentrated aqueous slurry containing carbamide
peroxide that is only partially solubilized in the aqueous
medium.
The particulate solid (carbamide peroxide in an aqueous medium or
carbamide peroxide injected as a solid) should have a relatively
small particle size in order to maximize the surface-to-volume
ratio, a factor that can enhance the effectiveness of the gas-solid
interaction between the NO.sub.X and solid and between the hydrogen
peroxide and solid. The mean particle size of the particulate solid
is preferably be less than about 100 .mu.m, more preferably less
than about 70 .mu.m, and most preferably less than about 40
.mu.m.
The advantages of carbamide peroxide as a component in the reagent
employed in this invention are numerous. Carbamide peroxide is a
solid, inert, non-corrosive material. It is easily handled and
safely stored. It provides a stable molar ratio
H.sub.2O.sub.2:Urea=1:1 in the flue gas at any concentration used.
This ratio was found optimal for NO.sub.X (e.g., NO) reduction.
The second important component of the reagent composition is water.
It is known that water participates in the radical reaction
initiated by hydrogen peroxide and thus is important for NOx
reduction. Thus, the water functions as an active component in the
reaction and not simply a carrier or a solute
The concentration of water can vary, but the molar ratio of water
to carbamide peroxide in the aqueous solution is preferably
maintained within a range of from about 5:1 to about 500:1, and
more preferably within a range of from about 50:1 to about 200:1
H.sub.2O:carbamide peroxide.
The aqueous reagent solution preferably contains at least 1 wt. %
total of the carbamide peroxide, more preferably at least about 3
wt. % total of t carbamide peroxide, and most preferably at least
about 5 wt. % carbamide peroxide. Higher total concentrations of
the carbamide peroxide in the aqueous reagent solution are
feasible, e.g., 10 wt. % or more or even 20 wt. % or more of
carbamide peroxide, with the recognition that very high total
concentrations (e.g., above 50 wt. %) may exceed the maximum
solubility of the carbamide peroxide, resulting in the formation of
an aqueous slurry.
Carbamide Peroxide in combination with Urea or H.sub.2O.sub.2
The molar ratio of urea to hydrogen peroxide in carbamide peroxide
is 1:1. To modify this ratio, solid urea or liquid aqueous solution
of H.sub.2O.sub.2 can be added to the aqueous solution of carbamide
peroxide. For example, the reagent used in this invention can
comprise a mixture of carbamide peroxide, water, and urea. The
molar ratio of the carbamide peroxide to the urea to the water can
be in the range of from about 0.25:1:50 to about 20:1:1000. In some
embodiments, the reagent used can comprise a mixture of carbamide
peroxide, water, and H.sub.2O.sub.2. The molar ratio of the
carbamide peroxide to the H.sub.2O.sub.2 to the water can be in the
range of from about 0.5:1:50 to about 10:1:500.
Urea (NH.sub.2CONH.sub.2) is a stable solid at room temperature.
Urea is water soluble and may be completely dissolved in water or
in aqueous solutions also containing carbamide peroxide, for
subsequent injection into a flue gas stream being treated in the
method of this invention. The concentration range of such
urea--hydrogen peroxide solutions may vary over a relatively wide
range, given that the maximum solubility of urea (or carbamide
peroxide) in aqueous media is relatively high.
The molar ratio of carbamide peroxide to urea in the aqueous
solution is preferably maintained within a range of from about
0.25:1 to about 20:1 carbamide peroxide to urea.
The hydrogen peroxide is preferably utilized as aqueous hydrogen
peroxide. It can be added to an aqueous solution of carbamide
peroxide, in various proportions, which allows to change the urea
to H.sub.2O.sub.2 ratio in the reagent used for NOx reduction.
The molar ratio of carbamide peroxide to H.sub.2O.sub.2 in the
aqueous solution is preferably maintained within a range of from
about 0.5:1 to about 10:1 carbamide peroxide to H.sub.2O.sub.2.
The molar ratio of urea to hydrogen peroxide in the aqueous
solution is preferably maintained within a range of from about
0.1:1 urea:H.sub.2O.sub.2 (i.e., 10:1 H.sub.2O.sub.2:urea) to about
5:1 urea:H.sub.2O.sub.2 (i.e., 1:5 H.sub.2O.sub.2:urea), and more
preferably within a range of from about 0.2:1 urea:H.sub.2O.sub.2
(i.e., 5:1 H.sub.2O.sub.2:urea) to about 3:1 urea:H.sub.2O.sub.2.
The molar ratio of urea to hydrogen peroxide is preferably at least
equimolar, 1:1 urea:H.sub.2O.sub.2. More preferably, the molar
ratio of urea to hydrogen peroxide provides a molar excess of urea
relative to the hydrogen peroxide. More preferably the amount of
urea present in the aqueous solution provides a molar excess of
urea with respect to the hydrogen peroxide, e.g., at least about
1.1 moles urea per 1 mole H.sub.2O.sub.2, up to about 3 moles urea
per 1 mole H.sub.2O.sub.2 and, more preferably, up to about
2:1:urea:H.sub.2O.sub.2.
Carbamide peroxide and urea solutions typically have a relatively
neutral pH. Aqueous solutions containing both urea and hydrogen
peroxide are slightly acidic, e.g., about pH 3.5 to about pH 5.5,
which promotes stability of the hydrogen peroxide against
decomposition.
Ammonia (the preferred reducing agent in conventional SCR and SNCR
NO.sub.X treatment operations), by contrast, lacks many of the
advantages associated with urea just mentioned. Ammonia is a
corrosive gas and in aqueous solution has a relatively alkaline pH,
11-12. In addition, aqueous solutions that contain both ammonia and
hydrogen peroxide are typically unstable, with the hydrogen
peroxide being susceptible to decomposition at room temperature due
to the alkaline solution pH. Ammonia also has a tendency to react
with SO.sub.3 in SO.sub.X- and NO.sub.X-containing flue gas
streams, leading to formation of ammonium sulfate, a corrosive and
sticky solid salt that can lead to plugging in flue gas stream
ductwork.
The concentration of hydrogen peroxide in the aqueous reagent
solution (containing urea and hydrogen peroxide) may vary over a
wide concentration range, from about 0.5 wt. % to about 50 wt. %
H.sub.2O.sub.2.
The preferred concentration range for the hydrogen peroxide
component in the aqueous reagent solution is desirably in the range
of from about 1 wt. % to about 40 wt. % H.sub.2O.sub.2, preferably
in the range of from about 2 wt. % to about 30 wt. %
H.sub.2O.sub.2, and more preferably in the range of from about 3
wt. % to about 20 wt. % H.sub.2O.sub.2. Most preferably, the
hydrogen peroxide component in the aqueous reagent solution is
present at a concentration of at least 5 wt. % H.sub.2O.sub.2.
The preferred concentration range for the hydrogen peroxide
component in the aqueous reagent solution is normally determined
indirectly, by the choice of (1) of the urea concentration desired
in the aqueous medium (a decision that normally takes into
consideration the maximum solubility of urea possible in an aqueous
solution). (2) the specific molar ratio of urea:H.sub.2O, that is
desired for the aqueous reagent solution and (3) the desired total
concentration of reagent components in the aqueous reagent
solution.
With regard to the third factor just mentioned, another
consideration in this decision process is the preference that the
aqueous reagent solution not be overly dilute with respect to the
total amount of each component present, since highly dilute
solutions require evaporation of significantly more water in the
flue gas stream. This can lead to cooling of the gas stream and an
undesired reduction in the gas stream temperature (i.e., the
treatment temperature).
The source aqueous hydrogen peroxide used in the present invention
may have a wide range of aqueous solution concentrations, with
aqueous solutions preferably containing about 10 wt. % to about 50
wt. % H.sub.2O.sub.2 and those containing from about 20 wt. % to
about 40 wt. % H.sub.2O.sub.2 being more preferred. Aqueous
hydrogen peroxide solutions within these concentration ranges are
readily available from commercial suppliers, as stabilized
H.sub.2O.sub.2 solutions.
Concentrations of aqueous H.sub.2O.sub.2 above 50 wt. %
H.sub.2O.sub.2 are feasible but require stringent handling and
safety measures and are best avoided for that reason.
Concentrations of aqueous H.sub.2O.sub.2 below 10 wt. %
H.sub.2O.sub.2 are likewise feasible but are relatively dilute,
requiring relatively larger volumes to provide the same amount of
H.sub.2O.sub.2 as provided in much smaller volumes of more
concentrated aqueous solutions.
The activity of the hydrogen peroxide as a reagent in combination
with the urea reagent component may optionally be enhanced or
increased, in the present invention, by the use of one or more
activators in conjunction with the aqueous hydrogen peroxide. The
activator may be added or otherwise introduced into the aqueous
solution containing urea and hydrogen peroxide shortly before that
aqueous solution is injected into the NO.sub.X-containing flue gas
stream or may be introduced concurrently with the aqueous reagent
solution during the injection procedure, provided that there is
good mixing between the two. The activator may alternatively be
added to the source aqueous hydrogen peroxide that is used to
prepare the aqueous solution containing urea and hydrogen
peroxide
Activators for hydrogen peroxide include metal ions (e.g., iron,
copper, manganese, chromium, nickel), metals (e.g., platinum,
silver) and metal compounds (e.g., oxides, hydroxides or sulfides.
e.g., of manganese, iron, copper, palladium). A preferred activator
is iron and, as is evident for the exemplified metals, transition
metals, including the heavy metals, are also preferred.
Combinations of metal activators may be used, with iron and copper
being a preferred synergistic combination.
Other materials that may be used as hydrogen peroxide activators in
the present invention include oxidizing agents such as ozone,
hypochlorite (e.g., sodium or calcium hypochlorite), chlorite
(e.g., sodium chlorite), chlorate (e.g., sodium, potassium, or
magnesium chlorate), and the like.
The hydrogen peroxide activator may be introduced into the aqueous
medium containing urea and hydrogen peroxide (or into the source
aqueous hydrogen peroxide) in dissolved form or in suspended form.
Small amounts of activator, in the range of parts per million, are
normally sufficient to enhance the hydrogen peroxide activity.
A noteworthy and surprising aspect of the present invention is that
hydrogen peroxide, when used in combination with the urea in the
two component reagent employed in this invention, appears not to
function as an oxidizer with respect to the NO.sub.X components in
the flue gas stream being treated. Hydrogen peroxide, a strong
oxidizer, will result in oxidation of NO to NO.sub.2 in a flue gas
stream containing NO.sub.X, if introduced into the gas stream as
the sole reagent; see Comparative Example 4, below.
Hydrogen peroxide, when used in combination with urea in the
reagent employed in the method of this invention, appears to
function as an activator or agent that enhances the reducing
functionality of the urea (with respect to the NO.sub.X in the flue
gas stream being treated), rather than oxidizing NO present in the
gas stream. In addition, the hydrogen peroxide in this unique role
enables the urea to provide its desired reducing functionality at
much lower flue gas stream temperatures than are conventionally
used with urea alone.
Aqueous Reagent Containing Urea and Hydrogen Peroxide
The aqueous reagent medium containing urea and hydrogen peroxide
that is employed as the reducing agent in the method of this
invention for reaction with NO.sub.X in flue gas streams may be
used over a wide range of concentrations, with respect to the total
amount of urea and hydrogen peroxide present in the aqueous
solution or medium.
With the use of the solid adduct carbamide peroxide as the source
of both urea and hydrogen peroxide for the reagent of this
invention, the 1:1 molar ratio of urea:H.sub.2O.sub.2 provided by
carbamide peroxide may be adjusted above or below this equimolar
amount via supplemental addition of urea or hydrogen peroxide.
The aqueous reagent solution is preferably within a temperature of
about 10.degree. C. to about 50.degree. C., more preferably about
15.degree. C. to about 30.degree. C., prior to its injection or
introduction into the flue gas stream. Higher or lower temperatures
outside of the preferred range are also feasible, but very low
solution temperatures may limit the maximum solubility of urea and
very high solution temperatures may result in unwanted
decomposition of the hydrogen peroxide, prior to injection of the
solution.
Reagent Reaction with NO.sub.X
The reaction between NO.sub.X, specifically NO, and the urea and
hydrogen peroxide in the reagent of this invention is believed to
occur as follows:
2NO+(NH.sub.2).sub.2CO+H.sub.2O.sub.2.fwdarw.2N.sub.2+3H.sub.2O+-
CO.sub.2 (1)
This reduction reaction converts NO to N.sub.2 and results in
formation of nitrogen (N.sub.2), carbon dioxide (CO.sub.2) and
water (H.sub.2O), all gaseous reaction products. These reaction
products are gaseous compounds that are typically present in a flue
gas stream from a stationary-source combustion source and thus do
not introduce any new species or unwanted reaction byproducts into
the treated gas stream. As such, the urea and hydrogen peroxide
reagent employed in this invention may be considered
environmentally friendly.
The amount of reagent introduced into the flue gas stream for
reaction with the NO.sub.X present in the gas stream is normally
controlled via the flow rate of reagent introduced into the gas
stream, to provide the desired amount of urea or hydrogen peroxide
relative to the NO to be reacted.
The amount of reagent introduced into the gas stream should provide
a minimal concentration of both urea and H.sub.2O.sub.2 in the gas
stream of at least 100 ppm, and minimal concentration of water
vapor in the gas stream should be at least 1000 ppm.
The amount (flow rate) of reagent introduced into the flue gas
stream is preferably adjusted to provide a molar ratio of urea:NO
(i.e., urea in the reagent:NO in the gas stream) within the range
of from about 0.1:1 urea:NO to about 10:1 urea:NO and more
preferably within the range of from about 0.2:1 urea:NO to about
5:1 urea:NO and most preferably within the range of from about
0.5:1 urea:NO to about 3:1 urea:NO.
The molar ratio of urea to NO is preferably slightly less than or
greater than stoichiometric (i.e., +/-20% of stoichiometric, which
is about 2:1 urea:NO--see reaction (1) above), so as to minimize
the formation of unwanted reaction byproducts, e.g., ammonia
(NH.sub.3) in the treated flue gas stream. More preferably, the
reagent is introduced into the flue gas stream in an amount that
provides at least a stoichiometric molar amount of urea for
reaction with the NO in the flue gas stream being treated.
The urea--hydrogen peroxide treatment method of this invention is
primarily directed to removal of NO.sub.X in the treated flue gas
stream and, as noted above, an excess of hydrogen peroxide (with
respect to the urea reagent component) may be employed in the
present invention. Any unreacted hydrogen peroxide excess that
remains after reaction of the urea with NO.sub.X is also available
to react with other contaminants in the flue gas stream, e.g.,
SO.sub.X, particularly SO.sub.3, converting them to less
objectionable or more readily removed species.
Reagent pH
The aqueous solution containing urea and hydrogen peroxide that is
employed as the reagent in the method of this range typically has a
solution pH that is slightly acidic, e.g., in the range of about pH
3.5 to about pH 5.5. As mentioned previously, the slightly acidic
pH values of aqueous solutions containing urea and hydrogen
peroxide is beneficial for promoting stability of the hydrogen
peroxide in solution against decomposition.
In contrast to the slightly acidic pH range of aqueous solutions
containing the urea-hydrogen peroxide reagent of this invention,
aqueous ammonia solutions (the preferred reducing agent
conventionally employed in SNCR and SCR NO.sub.X treatment systems)
are alkaline, having pH values in the range of about pH 11-12. The
pH of solutions containing both ammonia and hydrogen peroxide are
likewise alkaline, typically in the range of about pH 10-11, and
such alkaline pH ranges (unlike acidic pH values) are detrimental
to the stability of hydrogen peroxide.
Injection of Reagent into Flue Gas Stream
The aqueous solution (or aqueous medium) containing urea and
hydrogen peroxide may be injected into the NO.sub.X-containing flue
gas stream using conventional injection equipment normally used for
introducing an aqueous solution or aqueous slurry into a gas
stream.
The aqueous solution is preferably injected, i.e., introduced, into
the flue gas stream as fine liquid streams or, preferably, as a
spray, such as atomized droplets, through one or more spray
nozzles. The spray nozzles or other introduction means (e.g.,
perforated pipe) should be designed to provide uniform dispersal
and good mixing of the aqueous solution containing urea and
hydrogen peroxide into the NO.sub.X-containing flue gas stream, to
facilitate reaction with the gaseous NO.sub.X in the gas
stream.
The atomized spray of aqueous reagent solution is believed to be
quickly vaporized when contacted with the hot flue gas stream upon
being injected into the flue gas stream. The reaction between the
reagent (urea and hydrogen peroxide) and nitrogen oxide in the flue
gas stream is believed to occur very quickly, but the exact
mechanism of reaction is not known nor whether the reaction
involves (in part) absorption of NO into the liquid droplets
introduced into the flue gas stream during the injection
procedure.
In the case of extremely hot flue gas streams, the injection system
design should include provisions for ensuring that the hydrogen
peroxide contained in the aqueous solution does not become
prematurely heated or overheated (and vulnerable to decomposition)
prior to its introduction into the hot flue gas stream.
As mentioned earlier, the reagent components, urea and hydrogen
peroxide, are injected or otherwise introduced into the flue gas
stream concurrently and preferably in combination with each other.
The reagent is preferably in aqueous form, e.g., preferably as an
aqueous solution containing urea and hydrogen peroxide.
The urea and hydrogen peroxide reagent components are preferably
combined prior to their injection or introduction into the flue gas
stream, but, alternatively, the urea and hydrogen peroxide
components may be introduced separately but concurrently into the
flue gas stream being treated provided that good mixing of the two
reagent components is ensured. Separate aqueous streams containing,
respectively, urea and hydrogen peroxide may be introduced
concurrently into the flue gas stream being treated, with mixing of
the two components being effected in the flue gas stream.
Such concurrent introduction of the two component streams may be
accomplished using separate nozzle systems for each component or by
using individual nozzles that have separate internal channels for
spraying the two liquid streams concurrently as atomized droplets
of each which become intermixed in the flue gas stream. This
procedure of introducing the two component streams concurrently has
the advantage of permitting quick and easy adjustment of the molar
ratio of urea:H.sub.2O.sub.2, as may be needed, to compensate for
changes in the flue gas composition or other gas stream
characteristics.
The reagent may also be introduced into the flue gas stream being
treated in solid form, e.g., powdered or particulate carbamide
peroxide or urea, or as an aqueous medium that also contains
solids, e.g., as an aqueous slurry containing urea and hydrogen
peroxide (the slurry containing urea solids, dissolved urea and
aqueous hydrogen peroxide.)
Although the carbamide peroxide or urea components of the reagent
may be introduced as a powdered solid or as an aqueous slurry (in
particulate and partially solubilized form), they are preferably
introduced into the flue gas stream being treated as an aqueous
solution (completely solubilized) and more preferably is in an
aqueous solution containing urea and hydrogen peroxide.
The hydrogen peroxide is preferably utilized as an aqueous hydrogen
peroxide solution and is normally introduced into the flue gas
stream as such, preferably in combination with the urea in the same
aqueous solution.
Operating Temperature
The NO.sub.X treatment method of this invention using the
urea-hydrogen peroxide reagent is normally carried out with flue
gas stream temperatures ranging from about 300.degree. C. to about
800.degree. C., without need for a catalyst such as is used in
conventional SCR NO.sub.X operations that utilize a catalyst in
order to operate at temperatures below 800.degree. C. Flue gas
stream temperatures are preferably within the range of from about
350.degree. C. to about 750.degree. C., more preferably within the
range of from about 350.degree. C. to about 650.degree. C., still
more preferably within the range of about from 350.degree. C. to
about 600.degree. C., and most preferably within the range of from
about 400.degree. C. to about 550.degree. C. for urea-hydrogen
peroxide treatment method of this invention.
In general, special gas temperature adjustments (i.e., heating or
cooling steps) are not required for the flue gas stream as a
prerequisite of the urea-hydrogen peroxide NO.sub.X treatment of
this invention. The urea-hydrogen peroxide injection, in the
present invention, may be carried out by locating the injection
point in the combustion gas stream at a suitable point where the
gas stream temperature is within the desired temperature range,
e.g., upstream of an economizer and downstream from the combustion
unit in the flue gas stream ducting from an electric utility power
plant.
The method of the present invention may be carried out with
existing SNCR-treated flue gas streams, which are typically
operated at temperatures well in excess of 800.degree. C. Since the
inventive method allows operation at lower temperature ranges than
the very high temperatures conventionally used with SNCR
treatments, the flue gas stream may optionally be subjected to a
heat exchange step upstream of the SNCR operation to recover heat
values from the flue gas stream and thus effect a lowering of the
flue gas stream temperature to the preferred operating temperatures
ranges employed in this invention, e.g., within a temperature range
of from about 300.degree. C. to about 800.degree. C.
Residence Time
Residence time required for reaction of the urea-hydrogen peroxide
reagent and NO.sub.X, e.g., NO, after injection or introduction of
the reagent into the flue gas stream is typically very short, from
a fraction of a second, e.g., 0.01 second, to less than a few
seconds, e.g., up to about 5 seconds. Preferred residence times for
the injected reagent being in contact with the NO.sub.X-containing
gas stream are generally less than about 2 seconds. The optimum
residence time will normally depend on factors such as the
temperature of the flue gas stream, with higher gas temperatures
generally providing more rapid reaction.
NO Removal Efficiencies
The urea and hydrogen peroxide reagent employed in the method of
the present invention is normally capable of reducing the initial
NO concentration in the flue gas stream to significantly lower NO
concentrations in treated flue gas stream, such that the treated
gas stream has a reduced concentration of NO. Sufficient urea and
hydrogen peroxide reagent is desirably employed in the method of
this invention to reduce the NO concentration in the flue gas
stream to less than about 60% of its initial concentration, prior
to treatment according to this invention. Preferably, the urea and
hydrogen peroxide reagent treatment is sufficient to reduce the NO
present in the treated flue gas stream to less than about 50%, and
more preferably less than about 40%, of its initial concentration
in the flue gas stream prior to treatment.
The urea and hydrogen peroxide reagent employed in the method of
the present invention is preferably utilized in a selective
non-catalytic reduction operation to achieve these removal
efficiencies.
Optional Desulfurization
The present invention for reducing NO.sub.X in NO.sub.X-containing
flue gas streams may also be employed in conjunction with
desulfurization operations, for reducing the concentration of or
substantially removing SO.sub.X. e.g., SO.sub.2 and/or SO.sub.3,
that are usually also present in NO.sub.X-containing flue gas
streams.
Such optional desulfurization unit operations may be carried out
either upstream or downstream of the NO.sub.X treatment method of
the present. Preferably, the desulfurization is carried out on the
NO.sub.X-depleted flue gas stream at a location downstream of the
urea-hydrogen peroxide treatment method of the present invention.
This is particularly so in the case of wet desulfurization
operations being employed, since exiting flue gas stream
temperatures are significantly reduced upon passage through wet
scrubbers or absorbers.
The SO.sub.X in combustion flue gas streams is primarily sulfur
dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3). These SO.sub.X
components are normally formed during the combustion of
sulfur-containing (sour) fuels, such as coal, coke or oil, and the
flue gas streams that result from burning such sulfur-containing
fuels, whether low-sulfur or high sulfur, consequently contain
SO.sub.X contaminants.
Sulfur dioxide is the predominant SO.sub.X component in flue gas
streams, with sulfur trioxide. SO.sub.3, being produced in much
smaller quantities than SO.sub.2. Concentrations of SO.sub.2 in
flue gas streams from coal fired boilers are typically substantial,
e.g. about 0.01 vol. % to about 0.5 vol. % SO.sub.2, with about
0.05 vol. % to about 0.3 vol. % SO.sub.2 being typical.
Typical concentrations of SO.sub.3 in flue gas streams from coal
fired boilers are about 10 ppm to about 30 ppm (by volume)
SO.sub.3. Conventional pollution control operations to remove
NO.sub.X components from a flue gas stream, e.g., via selective
catalytic reduction (SCR), often result in an unwanted increased
concentration of SO.sub.3, formed by the catalytic oxidation of
SO.sub.2 in the flue gas stream during SCR treatment, to levels
that can double those normally present, e.g., to about 20 to about
60 ppm or more SO.sub.3. Likewise, the presence of catalytic
metals, e.g., vanadium or nickel, in some fuels can also result in
the generation of additional sulfur trioxide.
These SO.sub.X contaminants are desirably removed, or their
concentrations reduced, in the combustion flue gas stream via
desulfurization procedures, prior to the flue gas stream being
released into the atmosphere. Such desulfurization operations are
readily incorporated into an integrated air pollution control
system that utilizes the present invention for NO.sub.X removal, in
the treatment of a NO.sub.X- and SO.sub.X-containing combustion
flue gas stream.
Desulfurization processes for removing SO.sub.2 and/or SO.sub.3 are
well known in the air pollution control field. Gas-liquid
contactors or absorbers are widely used to remove SO.sub.2 from
waste flue gas streams, using an alkaline reagent-containing
aqueous medium, e.g., in wet scrubbing systems utilizing lime,
limestone or soda ash (sodium carbonate). Conventional techniques
for specific treatment of flue gas streams to reduce SO.sub.3
concentrations employ alkali reagents in wet scrubbing, slurry
injection or dry sorbent injection procedures. Some prior art
desulfurization procedures are effective for removing both SO.sub.2
and SO.sub.3.
The present invention may be adapted for use with many conventional
desulfurization systems, whether employed to remove SO.sub.X
components generally or SO.sub.2 or SO.sub.3 specifically. When
used in conjunction with the present invention, such
desulfurization systems are preferably located downstream, for
desulfurization of the NO.sub.X-depleted flue gas stream resulting
from treatment according to the present invention. Wet
desulfurization systems are preferred for use in conjunction with
the present invention, particularly wet scrubbing desulfurization
systems that employ lime, limestone or soda ash.
Upstream desulfurization may be desirable in situations where flue
gas streams contain high concentrations of SO.sub.3. Injection of a
dry soda-type sorbent or slurried soda-type sorbent can be used to
remove a significant portion of SO.sub.3 upstream of the
urea--hydrogen peroxide treatment of a NO.sub.X- and
SO.sub.X-containing flue gas stream according to the present
invention.
The following non-limiting EXAMPLES illustrate preferred
embodiments of the present invention.
EXAMPLE 1
Examples 1-3 utilized the following laboratory apparatus. The lab
set-up included a pre-heater consisting of a quartz tube heated by
a Lindberg tube furnace; an insulated heated tube reactor
(diameter: 10 mm, length: 1,200 mm); and auxiliary equipment
including a syringe pump, temperature controllers, gas flow meters,
gas distribution panel, cold trap, and mist catcher.
A gas mixture consisting of NO, O.sub.2, and N.sub.2 was introduced
as a gas stream first into the pre-heater and then into the
insulated heated tube reactor heated to a predetermined
temperature. The gas components of the gas mixture were sourced
from nitrogen gas, air, and NO in nitrogen (390 ppm NO in N.sub.2).
Concentrations of each component in the gas mixture were adjusted
by a flow rate meter.
An injection inlet was provided at a position at the entrance to
(upstream of) the insulated heated tube reactor, and aqueous
urea-H.sub.2O.sub.2 reagent solutions were injected into the
flowing gas stream through a stainless steel tube with a diameter
of 1.0 mm.
Concentrations of NO and NO.sub.2 (together constituting NO.sub.X)
in the effluent gas downstream of the insulated heated tube
reactor, at the tube reactor outlet, were measured by a
chemiluminescence NO.sub.X analyzer. Inlet gas compositions were
measured as the composition of the gas stream at the outlet
initially at the beginning of each study, prior to reagent being
introduced. The accuracy of the initial outlet composition readings
was also verified by bypassing the incoming gas stream directly to
the NO.sub.X analyzer, without passing the gas stream through the
preheater and reactor.
Example 1 studied the effect of using varying amounts of carbamide
peroxide reagent relative to the amount of NO in the gas stream.
Five different molar ratios of reagent to NO in the inlet gas
stream were used to study the reagent concentration effect
(relative to the NO) on the removal of NO from the gas stream.
In Example 1, the aqueous solution of the reagent was prepared by
dissolution of carbamide peroxide, a solid adduct of urea and
hydrogen peroxide, in water to prepare an aqueous solution that
contained 5.0 wt. % carbamide peroxide, i.e. corresponding to 3.2
wt. % urea and 1.8 wt. % H.sub.2O.sub.2.
The molar ratio of urea:hydrogen peroxide in this aqueous reagent
solution was 1:1 since urea and H.sub.2O.sub.2 are present in a 1:1
molar ratio in the carbamide peroxide adduct. Consequently, the
molar ratios of (i) urea to NO and of (ii) hydrogen peroxide to NO
were the same in each of the five runs for this Example 1 (see col.
1 of Table 2 below).
Temperature of the gas stream in the tube reactor was 465.degree.
C. The gas stream flow rate was 3.2 L/min.
Table 2 summarizes the results for the five operating runs, with
the tube reactor inlet and outlet NO concentrations being shown
along with the percentage NO reduction achieved.
Not shown in the tabulated data in Table 2 is the observation that
essentially no NO.sub.2 was formed in the treated gas stream, a
conclusion based on NO.sub.2 measurements of the outlet gas stream
(see Example 7, below).
TABLE-US-00002 TABLE 2 Molar ratio of urea:NO and Inlet NO Outlet
NO NO Reduction of H.sub.2O.sub.2:NO (ppm) (ppm) (%) 0.5 113 65 43
1.1 116 57 51 1.6 112 42 63 2.1 120 40 67 3.2 120 44 63 Note re
column 1: because the molar ratio of urea:H.sub.2O.sub.2 in the
carbamide peroxide adduct was 1:1, the molar ratio of urea:NO was
the same as that of H.sub.2O.sub.2:NO.
The data presented in the Table 2 show that the injection of
carbamide peroxide aqueous solution into gas stream provided
reduction of NO over a range of molar ratios reagent
(urea=H.sub.2O.sub.2):NO between 0.5:1 to 3.2:1.
The NO removal was good over the range of molar reagent to NO
ratios studied, but best removal efficiencies were obtained with a
molar excess of urea (=hydrogen peroxide) relative to the NO in the
gas stream.
EXAMPLE 2
Example 2 studied the effect of using three different
urea-to-hydrogen peroxide molar ratios on the removal of NO from a
gas stream. The apparatus used for Example 2 was the same as that
utilized in Example 1.
In Example 2, the urea (in solid form) or hydrogen peroxide (50%
H.sub.2O.sub.2 source solution) was combined with carbamide
peroxide and water to prepare aqueous solutions containing urea and
hydrogen peroxide in three different molar ratios: 1.5:1, 1:1 and
0.67:1 urea:H.sub.2O.sub.2 in the aqueous reagent solutions.
All of these aqueous solutions, however, contained 5.0 wt. %
urea--hydrogen peroxide reagent, the same solution concentrations
of urea and hydrogen peroxide (corresponding to 3.2 wt. % urea and
1.8 wt. % H.sub.2O.sub.2) as were used in Example 1.
The molar ratio of H.sub.2O.sub.2:NO was kept constant in Example 2
at 2:1 H.sub.2O.sub.2:NO, so only the urea was varied with respect
to the inlet NO concentration in these studies.
The temperature of the gas stream in the tube reactor was
405.degree. C., somewhat less than the 465.degree. C. used in
Example 1. The gas stream flow rate was 3.2 L/min., the same as
used in Example 1.
Table 3 summarizes the three operating runs, with the tube reactor
inlet and outlet NO concentrations being shown along with the
percentage NO reduction achieved.
TABLE-US-00003 TABLE 3 Urea/H.sub.2O.sub.2 Inlet NO Outlet NO NO
Reduction molar ratio (ppm) (ppm) (%) 0.67 122 65 47 1.0 120 53 56
1.5 121 46 62
The results shown in Table 3 indicate that the best NO reduction
was obtained using a molar excess of urea, relative to the hydrogen
peroxide.
EXAMPLE 3
Example 3 studied the effect of gas stream temperature on the
removal of NO from a gas stream, under otherwise fixed operating
conditions, using the same urea-to-hydrogen peroxide molar ratio
and hydrogen peroxide to NO molar ratio in each of the six
temperature studies.
The apparatus used for Example 3 was the same as that utilized in
Examples 1 and 2.
In Example 3, as in Example 1, the aqueous solution of the reagent
was prepared by dissolution of carbamide peroxide, a solid adduct
of urea and hydrogen peroxide, in water to prepare an aqueous
solution that contained 5.0 wt. % carbamide peroxide, corresponding
to 3.2 wt. % urea and 1.8 wt. % H.sub.2O.sub.2.
The molar ratio of urea to hydrogen peroxide in the aqueous reagent
solution was therefore 1:1 urea:H.sub.2O.sub.2, as was the case in
Example 1. The aqueous reagent solution was introduced into the gas
stream in an amount that provided 2 moles of hydrogen peroxide to
each mole of NO in the inlet gas stream. The ratio of urea to NO
was likewise 2:1 urea:NO.
In Example 3, studies were carried out at six different gas stream
temperatures ranging from 330.degree. C. to 545.degree. C.
Table 4 summarizes the six operating runs at gas temperatures
ranging from 330.degree. C. to 545.degree. C., with the tube
reactor inlet and outlet NO concentrations being shown along with
the percentage NO reduction achieved.
TABLE-US-00004 TABLE 4 Gas Stream Temperature Inlet NO Outlet NO NO
Reduction (.degree. C.) (ppm) (ppm) (%) 330 121 97 20 405 120 53 56
445 123 49 60 465 120 46 62 505 121 44 64 545 122 43 65
The results shown in Table 4 indicate that the good NO reduction
was obtained within the temperature range of 405.degree. C. to
545.degree. C., under the specific urea:H.sub.2O.sub.2 and
H.sub.2O.sub.2:NO molar ratios employed in these studies.
The NO removal obtained at the lowest temperature, 330.degree. C.,
was much lower than that obtained at temperatures above 400.degree.
C., which suggests that the hydrogen peroxide was not sufficiently
activated to providing the good NO removal results obtained at
higher gas stream temperatures.
EXAMPLE 4
Comparative Example 4 studied the effect of using hydrogen peroxide
alone, to determine its effectiveness in the removal of NO from a
gas stream.
The apparatus used for Comparative Example 4 was the same as that
utilized in Example 1. In Comparative Example 4, concentrated
aqueous hydrogen peroxide (50 wt. %) was dissolved in water to
obtain an aqueous hydrogen peroxide solution containing 5 wt. %
H.sub.2O.sub.2.
The aqueous hydrogen peroxide solution was injected into the
NO-containing gas stream at a rate that provided two moles
H.sub.2O.sub.2 per mole of NO in the gas stream.
The temperature of the gas stream in the tube reactor was
405.degree. C., and the gas stream flow rate was 3.2 L/min., the
latter being the same as used in Example 1.
FIG. 1 shows the results of this study in Comparative Example 4
using hydrogen peroxide alone, in a graph with the x axis showing
time (in minutes, beginning with the initial injection of the
reagent at t=0) and the y axis showing NO, NO.sub.2 and NO.sub.X
(i.e., NO+NO.sub.2) concentrations (in ppm) at the outlet of the
tube reactor.
The results in FIG. 1 indicate that the hydrogen peroxide was
oxidizing NO to NO.sub.2 (as would be expected in the presence of
an oxidizer), the opposite of the desired objective of reducing NO
to N.sub.2. It should be noted that the overall level of NO.sub.X,
the additive concentrations of NO and NO.sub.2, remained relatively
constant.
While N.sub.2 is considered an environmentally benign reaction
product, the NO.sub.2 reaction product formed in this study using
hydrogen peroxide alone as the reactant is normally considered to
be an objectionable contaminant, as is NO, in a flue gas
stream.
EXAMPLE 5
Example 5 was a laboratory study carried out under the similar
conditions and in the same apparatus as in Comparative Examples 4
(H.sub.2O.sub.2 alone) except that the reagent utilized in Example
5 was carbamide peroxide, to determine this reagent's effectiveness
in the removal of NO from a gas stream.
In Example 5, carbamide peroxide, the solid adduct of urea and
hydrogen peroxide containing a mole ratio of 1:1
urea:H.sub.2O.sub.2, was dissolved in water to obtain an aqueous
solution containing 5.0 wt. % carbamide peroxide, corresponding to
3.2 wt. % urea and 1.8 wt. % H.sub.2O.sub.2.
This aqueous urea and hydrogen peroxide solution was injected into
the NO-containing gas stream at a rate that provided 2 moles
H.sub.2O.sub.2 per mole of NO in the gas stream.
The temperature of the gas stream in the tube reactor was
405.degree. C., and the gas stream flow rate was 3.2 L/min., the
latter being the same as used in Comparative Example 4.
FIG. 2 shows the results of this study in Example 5 using carbamide
peroxide as the reagent, in a graph with the x axis showing time
(in minutes, beginning with the initial injection of the reagent at
t=0) and the y axis showing NO, NO.sub.2 and NO.sub.X (i.e.,
NO+NO.sub.2) concentrations (in ppm) at the outlet of the tube
reactor.
The results in FIG. 2 indicate that the urea and hydrogen peroxide
in the reagent apparently effected a reduction of NO to N.sub.2
(the desired objective). There was only a very small amount of
NO.sub.2 produced during the run, which strongly suggests that
virtually all of the NO being reacted was being converted to
N.sub.2 (which was not measured). It should be noted that the
overall level of NO.sub.X, the additive concentrations of NO and
NO.sub.2, decreased along with the removal of NO.
In contrast to the results obtained in Comparative Example 4, the
results for Example 5 demonstrate an unexpected and very
significant benefit to the use of a reagent containing carbamide
peroxide in effecting conversion of NO in a gas stream to innocuous
reaction products.
EXAMPLE 6
Comparative Example 6 studied the effect of using urea alone, to
determine its effectiveness in the removal of NO from a gas
stream.
The apparatus used for Comparative Example 6 was the same as that
utilized in Example 1. In Comparative Example 6, the urea (in solid
form) was dissolved in water to obtain an aqueous solution
containing 5 wt. % urea.
The aqueous urea solution was injected into the NO-containing gas
stream at a rate that provided two moles urea per mole of NO in the
gas stream.
The temperature of the gas stream in the tube reactor was
405.degree. C. The gas stream flow rate was 3.2 L/min., the same as
used in Examples 1-5.
The results of this study found that there was essentially no
change in the NO concentration between the inlet and outlet of the
tube reactor, indicating that the urea alone was ineffective for
removing or reducing or otherwise affecting the concentration of NO
in the gas stream at 405.degree. C.
The procedure was repeated for other gas temperatures, namely,
500.degree. C. and 625.degree. C., with similar outcomes. These
results indicated that urea alone was ineffective for removing NO
in NO-containing gas streams, particularly within temperature range
studied, 400.degree. C. to 625.degree. C.
FIG. 3 shows the results of this study in Comparative Example 6 for
the temperature 625.degree. C. using urea as the reagent, in a
graph with the x axis showing time (in minutes, beginning with the
initial injection of the reagent at t=0) and the y axis showing NO,
NO.sub.2 and NO.sub.X (i.e., NO+NO.sub.2) concentrations (in ppm)
at the outlet of the tube reactor. As can be seen from the data,
the effect of urea alone on the NO reduction was negligible at
625.degree. C.
It should be noted that these temperatures are significantly below
those normally utilized in commercial SNCR systems that treat
NO-containing gas streams, which is believed to explain the lack of
any NO removal at the temperatures used in these studies.
EXAMPLE 7
Example 7 was a laboratory study that evaluated and compared the
effectiveness of two reagents in the removal of NO from
NO-containing gas streams at temperatures of 625.degree. C.,
750.degree. C. and 800.degree. C. The two reagents were (1)
carbamide peroxide in aqueous solution and, for comparative
purposes. (2) urea alone, in aqueous solution.
Example 7 was a carried out under the similar conditions and in the
same apparatus as in Comparative Example 4 (H.sub.2O.sub.2 alone)
and 6 (urea alone). The gas stream flow rate was 3.2 L/min.
In this Example 7, the reagent was obtained by dissolution of
carbamide peroxide in water to obtain an aqueous solution
containing 5.0 wt. % carbamide peroxide, corresponding to 3.2 wt. %
urea and 1.8 wt. % H.sub.2O.sub.2, as was done in Example 5.
Likewise for this Example 7, the comparative urea reagent was
obtained by dissolution of urea (in solid form) in water to obtain
an aqueous solution containing 5 wt. % urea, as was done in
Comparative Example 6.
The respective reagents, aqueous urea--hydrogen peroxide solution
and aqueous urea solution, were injected into the NO-containing gas
stream at rates that provided 2 moles urea per mole of NO in the
gas stream (also corresponding to 2 moles H.sub.2O.sub.2 per mole
of NO in the case of the carbamide peroxide reagent.)
Temperature=625.degree. C.
FIG. 4 shows the results of the study in Example 7 at the
temperature of 625.degree. C. using aqueous carbamide peroxide
solution as the reagent, in a graph with the x axis showing time
(in minutes, beginning with the initial injection of the reagent at
t=0) and the y axis showing NO, NO.sub.2 and NO.sub.X (i.e.,
NO+NO.sub.2) concentrations (in ppm) at the outlet of the tube
reactor.
The results in FIG. 4 indicate that the urea and hydrogen peroxide
in the reagent reduced the NO concentration in the gas stream at
625.degree. C. from an initial value of about 157 ppm NO to about
44 ppm NO, as shown by the data obtained after six minutes into the
run, and to about 80 ppm NO (a 72% reduction), as shown by the data
obtained after 12 minutes into the run.
Negligible NO.sub.2 was produced during this run at 625.degree. C.
(see the lower curve in FIG. 4), which strongly suggests that
virtually all of the NO being reacted was being converted to
N.sub.2 (which was not measured). It should be noted that the
overall level of NO.sub.X, the additive concentrations of NO and
NO.sub.2, decreased along with the removal of NO.
FIG. 3 shows the results of the comparative study in Example 6
using aqueous urea solution as the reagent at the same temperature
of 625.degree. C. A comparison of the data in FIG. 3 and FIG. 4
shows that urea alone cannot provide reduction of NO at this
temperature, whereas carbamide peroxide demonstrated a successful
NO reduction.
Temperature=725.degree. C.
FIG. 5 shows the results of the comparative study in Example 7
using aqueous urea solution as the reagent at the third temperature
of 725.degree. C., in a graph with the x axis showing time (in
minutes, beginning with the initial injection of the reagent at
t=0) and the y axis showing NO, NO.sub.2 and NO.sub.X (i.e.,
NO+NO.sub.2) concentrations (in ppm) at the outlet of the tube
reactor.
The results of this study shown in FIG. 5 demonstrated that there
was only a slight decrease in the NO concentration between the
inlet and outlet of the tube reactor. The NO concentration was
reduced from an initial value of about 145 ppm NO to about 125 ppm
NO (a 14% reduction), as shown by the data obtained after nine
minutes into the run at 725.degree. C.
These results indicate that the urea alone was ineffective for
achieving a significant reduction in the NO concentration of the
gas stream at 725.degree. C. The lower curve in FIG. 5 indicates
that no NO.sub.2 was formed during this study.
Temperature=800.degree. C.
FIG. 6 shows the results of the comparative study in Example 7
using aqueous urea solution as the reagent at the temperature of
800.degree. C., in a graph with the x axis showing time (in
minutes, beginning with the initial injection of the reagent at
t=0) and the y axis showing NO, NO.sub.2 and NO.sub.X (i.e.,
NO+NO.sub.2) concentrations (in ppm) at the outlet of the tube
reactor.
The results of this study shown in FIG. 6 demonstrated that there
was only a moderate decrease in the NO concentration between the
inlet and outlet of the tube reactor. The NO concentration was
reduced from an initial value of about 140 ppm NO to about 90 ppm
(a 36% reduction) after 12 minutes into the run, as shown by the
data in FIG. 6.
FIG. 7 shows the results of the study in Example 7 at the
temperature of 800.degree. C. using aqueous carbamide peroxide
solution as the reagent, in a graph with the x axis showing time
(in minutes, beginning with the initial injection of the reagent at
t=0) and the y axis showing NO, NO.sub.2 and NO.sub.X (i.e.,
NO+NO.sub.2) concentrations (in ppm) at the outlet of the tube
reactor.
The results in FIG. 7 indicate that the urea and hydrogen peroxide
in the reagent effected a significant reduction in the NO
concentration in the gas stream at 800.degree. C., much more
dramatic than the results in the previous runs at lower
temperatures. The carbamide peroxide aqueous reagent reduced the NO
in the gas stream from an initial value of about 140 ppm NO to
about 10 ppm (a 93% reduction) after 12 minutes into the run, as
shown by the data in FIG. 7.
Negligible NO.sub.2 was produced during this run at 800.degree. C.
(see the lower curve), which strongly suggests that virtually all
of the NO being reacted was being converted to N.sub.2 (which was
not measured). It should be noted that the overall level of
NO.sub.X, the additive concentrations of NO and NO.sub.2, decreased
along with the removal of NO.
These results indicate that the urea alone (FIG. 6) was ineffective
for achieving a significant reduction in the NO concentration of
the gas stream at 800.degree. C., especially when compared with the
results obtained with the carbamide peroxide aqueous reagent at
800.degree. C. (FIG. 7).
EXAMPLE 8
Example 8 studied the effect of water concentration in the gas
stream on the NO reduction efficiency. Five different molar ratios
of water to reagent in the inlet gas stream were used to study the
water concentration effect (relative to the carbamide peroxide) on
the removal of NO from the gas stream.
The apparatus used for Example 8 was the same as that utilized in
Examples 1-7.
In Example 8, as in Example 1, the aqueous solution of the reagent
was prepared by dissolution of carbamide peroxide in water to
prepare an aqueous solution that contained different concentrations
of carbamide peroxide.
The molar ratio of urea to hydrogen peroxide in the aqueous reagent
solution was therefore 1:1 urea:H.sub.2O.sub.2, as was the case in
Example 1. The aqueous reagent solution was introduced into the gas
stream in an amount that provided 2.1 moles of hydrogen peroxide to
each mole of NO in the inlet gas stream. The ratio of urea to NO
was likewise 2.1:1 urea:NO.
In Example 8, studies were carried out at five different
water:reagent molar ratios ranging from 5:1 to 200:1.
Table 5 summarizes the five operating runs at gas temperature of
465.degree. C., with the water to reagent molar ratios being shown
along with the percentage NO reduction achieved.
TABLE-US-00005 TABLE 5 Reagent/NO Water/reagent Temperature NO
Reduction molar ratio molar ratio (.degree. C.) Efficiency, % 2.1 5
465 11 2.1 15 465 30 2.1 50 465 48 2.1 100 465 67 2.1 200 465 71 *
Reagent is carbamide peroxide ** All other process parameters were
identical.
The data in the Table 5 suggest that water indeed plays an active
role in the NO reduction. Percentage of NO reduction increased with
increase of water to reagent molar ratio.
EXAMPLE 9
Example 9 studied the typical gas compositions in the reduction
experiments using carbamide peroxide. For five runs at different
experimental conditions the concentrations of injected urea,
hydrogen peroxide, and water in the gas stream were calculated. NO
concentrations in the inlet and outlet gas stream were measured and
NO reduction was determined.
Table 6 summarizes the five runs at different conditions, with the
reagent concentration of 5.0 wt %. The concentrations of the
components in the gas stream are shown along with the percentage NO
reduction achieved.
TABLE-US-00006 TABLE 6 Reagent NO concen- Concentration in the Gas
Stream Temper- Reduction tration, NO, Urea, H.sub.2O.sub.2,
H.sub.2O, ature Efficiency, wt. % ppm ppm ppm vol % (.degree. C.) %
5.0 120 240 240 2.4 330 20 5.0 120 240 240 2.4 465 56 5.0 120 250
250 2.5 465 67 5.0 110 180 180 1.8 465 63 5.0 115 125 125 1.3 465
51 *Reagent is carbamide peroxide ** All other process parameters
were identical.
The data in the Table 6 suggest that concentration of the active
species (Urea and H.sub.2O.sub.2) in the gas stream in all our
Examples were higher than 100 ppm. Generally, higher concentrations
of carbamide peroxide provide a more efficient removal of NO from
the gas stream.
EXAMPLE 10
Example 10 illustrates the application of a preferred embodiment of
the present invention to the NO.sub.X treatment of a flue gas
stream from a combustion boiler utilizing high sulfur coal. The
process is operated continuously, and normal steady state
conditions are assumed for purposes of Example 10. FIG. 8
illustrates a schematic flow diagram of this preferred embodiment;
reference numerals and letters in FIG. 8 are included in the
process description which follows. References to gaseous component
concentrations in percentage (%), parts per million (ppm) or parts
per billion (ppb) refer to such concentrations on a volume
basis.
The coal used in the combustion unit of this Example 10 is high
sulfur coal containing 2 wt. % sulfur. The combustion furnace is
operated with preheated air, and it is assumed that there is 1%
conversion of the sulfur in the coal to SO.sub.3 in flue gas from
the combustion unit. The exit combustion flue gas stream 1 contains
about 420 ppm NO.sub.X, i.e., 400 ppm NO and about 20 ppm NO.sub.2,
and also about 900 ppm SO.sub.2 and about 9 ppm SO.sub.3.
Referring now to FIG. 8, the combustion flue gas stream 1 is passed
through a heat exchange unit (not shown in FIG. 8) that reduces the
temperature of the hot combustion flue gas stream 1 to about
900.degree. F. (482.degree. C.). The reduced-temperature flue gas
stream 1 has essentially the same composition as prior to its being
subjected to a heat exchange unit operation.
The flue gas stream 1 is then treated in a selective non-catalytic
reduction reactor A to reduce its NO.sub.X content. The flue gas
stream is reacted in the selective non-catalytic reduction (SNCR)
unit operation with an aqueous reagent solution 2 that contains
water and carbamide peroxide. The aqueous reagent solution 2
contains 6.4 wt. % urea and 3.6 wt. % hydrogen peroxide (providing
a mole ratio of 1:1 urea:H.sub.2O.sub.2 in the reagent solution)
and is prepared (not shown in FIG. 8) by dissolution of appropriate
amounts of solid carbamide peroxide in water.
The aqueous reagent solution 2 containing water and carbamide
peroxide is introduced into the flue gas stream 1 via a set of
injection nozzles (not shown in FIG. 8) that spray and disperse the
solution into the hot flue gas stream in the SNCR reaction
apparatus A. Sufficient aqueous reagent solution 2 is introduced
into the flue gas stream 1 in the SNCR reactor A to provide 2 moles
of urea per mole NO in the flue gas stream. The amount of urea in
the injected aqueous reagent solution 2 thus provides a
stoichiometric amount of urea required to react with the NO that is
contained in the flue gas stream 1.
The reduction reaction of the urea--hydrogen peroxide reagent
solution 2 with NO.sub.X in the flue gas steam 1 in the SNCR
reactor A reduces the NO content of the flue gas stream 2,
producing N.sub.2, CO.sub.2 and water in the flue gas stream 3 that
exits the SNCR reactor A. The flue gas stream 3 exiting from the
SNCR unit operation A contains about 900 ppm SO.sub.2 and about 9
ppm SO.sub.3 and reduced levels of NO.sub.X, about 150 ppm
NO.sub.X.
The flue gas stream 3 exiting from the SNCR reactor A is directed
to one or more electrostatic precipitators (ESP), shown as block B
labeled as ESP in FIG. 8, to remove entrained solids, i.e.,
particulates such as fly ash from the coal combustion, from the
flue gas stream 3. The solids-free ESP-treated flue gas exits the
electrostatic precipitator operation B as flue gas stream 4. The
ESP solids, removed as stream 5, are disposed of in a landfill.
The ESP-treated flue gas stream 4 has a reduced NO.sub.X content,
as compared with the combustion flue gas stream 1 upstream of the
SNCR reactor A: the flue gas stream 4, downstream of the ESP
operation B, contains about 150 ppm NO.sub.X, compared to about 420
ppm NO.sub.X in the untreated combustion flue gas stream 1 upstream
of the SNCR reactor A.
The SO.sub.X-containing flue gas stream 4 exiting from the ESP
operation B is next subjected to a wet scrubbing desulfurization
procedure (not shown in FIG. 8) to reduce its SO.sub.2 and SO.sub.3
content before the flue gas stream 4 is vented to the atmosphere.
Wet desulfurization scrubbing operations using an alkali such as
lime, limestone or soda ash, are well known procedures for
desulfurizing SO.sub.X-containing flue gas streams.
It will be appreciated by those skilled in the art that changes
could be made to the embodiments described above without departing
from the broad inventive concept thereof. It is understood,
therefore, that this invention is not limited to the particular
embodiments disclosed but is intended to cover modifications within
the spirit and scope of the present invention as defined by the
appended claims.
* * * * *